This invention pertains generally to fluid flow pipelines and more particularly to a system and method for interrupting the flow of liquid in a pipeline in the event of a break or leak in the line.
It has been found that a sudden leak or break in a pipeline carrying liquid, such as petroleum products, under pressure is accompanied by a rapid drop in static pressure which is propagated away from the leak or break as a negative pressure wave. Such negative pressure waves travel through the flowing liquid with a velocity corresponding to the velocity of sound in the liquid. Heretofore, such negative pressure waves have been utilized for detecting and locating the position of a leak or break in the pipeline.
Pipelines used for petroleum products and other liquids are commonly provided with pumping stations and valves spaced along the line for pressurizing the liquid and controling the flow in the line. One or more of the valves can be closed to isolate a portion of the line in which a leak or break occurs, and in many instances it would be desirable to have a valve close automatically in the event of a leak or break.
If it were not for certain transient disturbances which tend to occur in the normal operation of a pipeline carrying liquid, it would be feasible to utilize the negative pressure wave which accompanies the leak or break in the line as a criterion for initiating automatic valve closing to isolate the leak or break. Such disturbances tend to occur, for example, when the pump at a pumping station is shut down or a control valve is closed or operated rapidly, and these disturbances are accompanied by negative pressure waves similar to those which accompany line breaks. In view of the difficulty of determining whether a negative pressure wave is generated by a leak or by normal pipeline operation, negative pressure waves are generally not used as the criteria for initiating automoatic valve closing in liquid lines.
It has been found that a negative pressure wave or rapid drop in static pressure can be utilized in an automatic valve control system for liquid pipelines if the response of the system is limited to negative waves originating on the downstream side of the system. In this regard, it has been noted that while pumping stations located along a pipeline produce high output pressures, their input suction pressures are relatively low. In normal operation, the suction pressure remains fairly constant, and any negative pressure waves generated by a pumping station are generally limited to the portion of the pipeline which is downstream of the pumping station. By limiting the operation of the valve control system to negative pressure waves originating downstream of the system, the problem of shutting the pipeline down in response to the action of a pump or valve in the normal operation of the line is effectively eliminated.